Currently, there are several types of switch mode converters and inverters, which are widely used for DC-to-DC, DC-to-AC, AC-to-DC and AC-to-AC power conversion. Currently, there are loads, the operation of which is optimized and efficiency maximized, if driven by special drive signal. For example, High Intensity Discharge (HID) lamps need to be driven by a low frequency AC signal, because high-frequency drive signal may destabilize the lamp's arc due to existence of acoustic resonance, which is a known phenomenon in the art. Accordingly, an inverter driving an HID lamp must have a current source nature (as opposed to voltage source nature) such that its characteristics contribute to the stability of the lamp's arc. One way to implement a low-frequency driver is to utilize electromagnetic ballast that is based on a large inductor, which is placed in series with the power line voltage. An alternative and preferred approach is to generate the low frequency signal by a switch mode inverter. A typical prior art solution is illustrated in FIG. 1.
FIG. 1 depicts a line rectifier (1), a power factor correction section (PFC), a buck converter that comprises a power switch QB, a stirring diode D1, an inductor Lf and a filtering capacitor Cf. The buck converter is controlled to operate as a current source by utilizing a feedback loop (not shown). The controlled DC current (I) is then fed to a commutator that is implemented by a full-bridge inverter (Q1 to Q4), and, therefore the lamp is driven by AC signal. Ignitor 2 is normally placed in series with lamp 3, in order to allow providing to the lamp the high-voltage spike that is required for its ignition phase. The circuit's configuration shown in FIG. 1 fulfills the lamp requirements in terms of ignition and low-frequency AC current. However, this implementation is rather expensive since it requires 5 power switches (in addition to the switches in the PFC circuitry) and an ignitor. Another drawback of this implementation is, that the power transistors are ‘hard-switched’ (i.e., they are switched between states while being under voltage) and, therefore, will have high switching losses. The problem of switching losses associated with the Buck section (i.e., Qb) is acute, because the Buck switch should preferably be operated at a high switching frequency. Another drawback associated with the configuration of FIG. 1 is that the spike-type ignition voltage restricts the distance between the ballast and the lamp, because short pulses, such as an ignition voltage pulse, decay rather fast as a function of their travel distance.
FIG. 2 (prior art) shows another solution for HID lamp ballast. Lamp 3 is driven by a high frequency signal generated by a half-bridge inverter (Q7, Q8). Ignition is accomplished by resonant circuit Cr and Lr. In order to ignite the lamp, the half-bridge inverter is driven by a frequency that is slightly higher than the resonant frequency fr:
      f    r    =      1          2      ⁢      π      ⁢                                    L            r                    ⁢                      C            r                              The resonant circuitry generates a high voltage across Cr, which ignites lamp 3. Once lamp 3 is ignited, the frequency of the drive signal is changed to fs in order to maintain the required magnitude of the lamp's current. The circuit's configuration shown in FIG. 2 is simpler and less expensive than the circuit's configuration shown in FIG. 1, and the transistors (i.e., in FIG. 2) are ‘soft-switched’ (i.e., switched under zero-voltage condition). However, the configuration shown in FIG. 2 suffers from several drawbacks. One drawback is associated with acoustic resonance, which usually develops within the cavity of the lamp whenever HID lamps are driven by high-frequency signals. Acoustic resonance normally causes arc instability, rupture/collapse of the arc and even explosion of the lamp. Although several methods have been suggested to overcome the acoustic resonance problem, for example frequency modulation and automatic frequency shifts, none of them has proven to be efficient for various types of lamps.
According to one aspect, the bridge depicted in FIG. 2, comprising Q5 to Q8, is driven by utilizing a combination of high and low frequency signals. For example, during the period of the first half of each cycle of the low-frequency signal, Q8 is switched into its conductive state, while Q5 and Q6 are driven by the high-frequency signal. Accordingly, the lamp current originates from Q5 and Q6. During the period of the second half of each cycle of the low-frequency signal, Q6 is switched into its conductive state, and Q7, Q8 are driven by the high-frequency signal, causing the lamp current to flow in opposite direction. Therefore, by utilizing this type of control method, the lamp can be driven, during its normal operating state, by a low frequency current. However, the latter control method has a drawback, being associated with the complicated control circuitry that is required for such implementation. In addition, the latter control method involves utilizing four power switches, which is another drawback. Furthermore, as would be apparent to a person skilled in the art, the latter power switches are switched under hard switching conditions (i.e., switched under excessive voltage), thereby causing to significant switching losses.
Another major drawback of the ballast shown in FIG. 2 is associated with the fact that the resonant circuit, used to generate the high voltage for ignition, is driven by a voltage source (the bus capacitor CBUS). Whenever the drive frequency is close to resonance frequency, the resonant circuitry (i.e., Lr and Cr) introduces essentially a zero ohmic resistance. Consequently, very high currents may develop, which may damage the apparatus. Accordingly, a special protection circuitry is required, that will allow providing to the lamp the high voltage that is required for its proper ignition, while guarantying that the resonant current is maintained at safe magnitude.
Another major drawback of the ballast shown in FIG. 2 is associated with ‘hot ignition’ phase of the lamp. The practical maximum voltage that is developed by a resonant circuit, such as shown in FIG. 2, (Lr, Cr), is insufficient for igniting hot HID lamps (i.e., ‘hot ignition’), because hot ignitions involve delivering to the lamp very high instantaneous voltages, i.e., between 15 kV and 45 kV. Therefore, an extra ignitor is required for generating the high voltage. Such extra ignitors are affiliated into conventional apparatuses as extra modules, which are placed in series with the lamp, causing additional complications and cost.
All of the methods described above have not yet provided a simple and efficient way for providing to an HID lamp the optimized power required for its ignition phase, whether ‘cold ignition’ or ‘hot ignition’, as well as for its normal (i.e., ‘steady-state’) operation.
There is thus a widely recognized need for electronic ballast for HID lamps that will have less power switches and will produce a low frequency AC current to drive the lamp. It would be also desirable that the same circuit be capable of producing the high-voltage required for the lamp's ignition phase, while self-regulating the maximum current of the power switches during ignition. It would be also advantageous to have an apparatus, which would be capable of generating the high voltage that is required also for hot-ignitions of HID lamps.
It is an object of the present invention to provide a method and apparatus for providing low-frequency AC current to electric loads, such as HID lamps, with improved efficiency.
It is another object of the present invention to provide a method and apparatus for providing efficient AC ‘cold’/‘hot-ignition’ current that are required to the operation of electric loads.
It is yet another object of the present invention to provide low-frequency AC current to electric loads, using soft switching.
It is still another object of the present invention to extend the reliability of electric loads, such as HID lamps.
Other objects and advantages of the invention will become apparent as the description proceeds.